The Bureau of Land Management (BLM), General Land Office (GLO) Records web site provides access to Federal land conveyance records for the Public Land States and image access to more than two million Federal land title records for Eastern Public Land States issued between 1820 and 1908.
This site offers researchers a source of information on the initial transfer of land titles from the Federal government to individuals in 30 states. In addition to verifying title transfer, this information will allow the researcher to associate an individual (Patentee, Assignee, Warrantee, Widow, or Heir) with a specific location (Legal Land Description) and time (Issue Date). [Note that the BLM does NOT hold the land records of 20 other states: the original 13 colonies, the 5 states created from the original 13 (Maine, Vermont, West Virginia, Kentucky, and Tennessee), plus Hawaii and Texas.]
The Pennsylvania Genweb has images of the organization of the various counties online. Click on the thumbnail images to see a larger version. They also have a clickable county map which will take you to a treasure trove of county resources.
Excellent year-by-year animated Pennsylvania county formation maps courtesy of AniMap show how the westward migration caused county formation. Click "Play" to run the "County Formation" program continuously or advance it one step at a time.
One of the hardest tasks of a genealogist is to figure out where their ancestors came from prior to where they settled. Look at the map on Migration Trails in Pennsylvania which can give you hints about counties you should search as you go back in time. In our own genealogical research experience, for example, a frequent migration path of early settlers in Clarion County was from Chester/Philadelphia > Berks County > Northampton County > Westmoreland and Centre Counties (yes, one group going to Westmoreland and another into Centre County) > Venango County > Clarion County.
For information regarding census records in the U.S. as well as the United Kingdom and Canada, check out Census Finder, a comprehensive portal to census records. The site is dedicated to finding sites which offer free census records online, so check it out!
Finally, I can't resist adding two extremely valuable sites for those people with ancestors in western PA, especially Somerset County, who went to Perry Co., Ohio. Tim Fisher, the creator of the sites, put together an index for the Lower Turkeyfoot Township Warrant Map of Somerset Co. which we have gratefully posted on our Somerset Co. page. After seeing the astonishing amount of material he has posted on his "Perry County Families" and "Perry County, Ohio" sites (including plat maps from 1846, 1859, and 1935), I asked him how much time he has spent on posting the information. Here's his response:
"You don't want to know. Just scanning, trimming, adjusting, etc. etc.
the 1850 census I figure took me over 100 hrs. The 1883 book took months.
I started researching in about '74, a year or so before the Roots series
on TV. My grandmother was Treasurer of Perry Co., OH for almost 35
years (the last 4 year term she was elected to, she was over 70 years old, but nobody knew it) and she became the defacto place for folks to stop in the courthouse to talk about their familes. Hers was the Goodin
family that came through Somerset Co., PA from Snyder Co., PA. Before
that we don't know where they were for sure. She had just about every
genealogy source there was to have for Perry Co., OH and I inherited them and have put most of them on the web.
The websites you see know are my 3rd or 4th generation. I've finally
gotten them about where I want them."